The present invention relates to an apparatus and method for measuring torque. Specifically, the present invention is an apparatus and method for the measurement of torque in rotating shafts. Particularly, the invention is a non-contacting, inductively coupled, method and apparatus for sensing torque induced leakage flux changes in a rotating ferromagnetic rod or shaft. The invention has particular application in the measurement of torque in a power steering system of an automotive vehicle.
In the field of mechanical instrumentation, the measurement of torque in rotating shafts has always been difficult. Torque is either measured indirectly, as a function of power and speed, or measured directly. Large machines, operating at relatively high torques, and very small machines, operating at relatively low torques, preclude the use of indirect torque sensing means, thus requiring a direct measurement technique. In other areas as well, direct measurement is preferred. However, direct measurement of torsion in rotating shafts can only be accomplished by measuring the actual strain in the shaft or by measuring the reaction torque (moment) with respect to a stable reference platform, which is difficult in certain situations. Thus, an improved method and means for direct measurement of torsion in a rotating shaft is needed.
Conventional methods for the direct measurement of shaft torsion can generally be grouped into two categories: contacting and non-contacting.
Contacting methods of measuring torque, such as the application of strain gages to the strained member, are common. Strain gages are very reliable, convenient and economical sensors. However, they function best when used in conjunction with stationary members. When applied to a rotating shaft, wires connecting the gages must be run through slip rings to the source of excitation, detection and amplification. Slip rings are notoriously noisy (electrically), subject to wear, and expensive to apply. The current state of the art in rotating strain gage torque sensors employs rotating transformers to induce an A.C. excitation signal to a strain gage bridge (rossette), and then, by using a second inductively coupled winding, transfers the bridge output signal to the remote electronics.
Variants of such transformer coupled strain gage torque sensors are currently commercially available, at rather high cost. The complexity and cost of such rotary transformer instruments relegates their application to laboratory or test-stand environments.
A common non-contacting method of measuring torque generally involves measurement of magnetic properties. The change of magnetic properties of various alloys as a result of an induced stress is well known. In particular, the premeability of a magnetic material tends to increase due to tensile stress and tends to decrease due to compressive stress. This effect has been utilized in some torque transducers. For example, in commonly assigned U.S. Pat. No. 4,414,855 the change in permeability of a magnetic layer on the surface of a non-magnetic cylindrical rod is sensed by one or more pick-up coils located adjacent to a stressed rotating rod. For a given pick-up coil, the inductance of the coil is directly proportional to the permeability of the magnetic layer. Since the permeability of the magnetic layer is directly proportional to the stress applied thereto, the inductance of the pick-up coils is directly proportional to the stress applied to the magnetic layer. Thus the stress applied to the cylindrical rod including the magnetic layer thereon may be determined by detecting the inductance of the pick-up coils. However, temperature dependence of these same magnetic properties restricts the usefulness of such non-contacting sensors. Fabrication of power transmitting shafts using such alloys also presents many difficulties.
When a metallic member is magnetized, a leakage flux is generated at any point where a discontinuity, flaw, or defect in the material exists. The quantity of leakage flux, and therefore the sensitivity of sensing devices to the defect, is dependent upon the relative orientation of the defect and the field. The present invention uses such leakage flux principles and is based upon the well known technique of eddy current testing, wherein discontinuities, cracks, inclusions or other defects in metallic objects are detected by means of changes in the flux due to induced current flow. Eddy current testing is primarily used as a sorting method or as a quality assurance tool.
Specifically, eddy currents are typically generated within an object to be inspected by induction from an adjacent coil establishing an alternating excitation current. The eddy currents then generate magnetic fields which couple to the coil at the same frequency as that of the excitation current, but which may be of a different phase. The phase and amplitude of the induced voltages depend upon the structural characteristics of the object under test. The phase relationships may be measured by appropriate signal processing circuits.
The flow of eddy currents in a test object is governed by the skin effect phenomenon. The currents decrease exponentially with depth, depending on the shape of the object, its thickness, and its electromagnetic properties. In addition to the decrease of current amplitude as depth below the surface increases, the phase angle of the current increasingly lags the excitation signal. While eddy current testing has been used in the prior art, the present invention, however, applies the eddy current testing concept in a novel manner for achieving a more useful, more reliable, more sensitive torque sensor.
Another important limitation of prior art torque sensing devices with measure stress-induced material property variations in a rotating shaft, is the insensitivity of torque direction. Whether the torsional member is stressed clockwise or counterclockwise, the net induced material property change will be identical--at least in a perfectly elastic system. For many applications, such as automotive steering effort sensors, the sign (direction) of the applied torque is essential information.